1. Field of the Invention
The present invention relates generally to television receivers and, more particularly, is directed to an automatic fine tuning (AFT) circuit for use with a television receiver in which the AFT operation can be readily pulled in.
2. Description of the Prior Art
As a channel selecting apparatus of, for example, a television receiver, a synthesizer tuner having a phase locked loop (PLL) circuit is utilized frequently. Such a synthesizer tuner can stably receive a television signal if the frequency of the television signal is inherently accurate.
Recently, it is frequently observed that the frequency of an RF (radio frequency) signal in the re-transmission of television signal in a so-called CATV, a video tape recorder (VTR), a television game machine or the like is offset from the original frequency. Further, in the CATV, in order to avoid the disturbance from the adjacent channel or the like, the frequency of the RF signal is offset from the original frequency by about 1 MHz.
Therefore, it is proposed to provide a so-called automatic fine tuning (AFT) circuit in the synthesizer tuner to pull-in the receiving signal having an offset frequency (see U.S. Pat. No. 4,422,096). Further, Japanese Patent Laid-Open Gazette No. 63-174483 describes a wide AFT in which the AFT operation is sequentially carried out at five to six channel-selection points within a channel so as to substantially widen the range in which the AFT operation is pulled-in.
In the above-described AFT circuit, in the stationary state, an AFT signal detecting period is restricted within only a vertical blanking period, whereby a fluctuation of the AFT signal caused by a level change of the video signal is removed, thus stabilizing the AFT operation (see Japanese Patent Laid-Open Gazette No. 63-135075).
However, under the condition that the synchronization of a receiving signal is not established, such as when a television channel is selected, the vertical blanking period of the receiving signal and an AFT detecting period, determined at the television receiver side, are not coincident with each other. In that case, if the receiving signal is a video signal of normal level, a satisfactory AFT control signal is obtained in a period other than the vertical blanking period.
If, for example, the receiving signal is a video signal having a high average picture level (APL), the level of the IF (intermediate frequency) signal is lowered during the video signal period so that the level of the AFT signal is lowered. Consequently, an AFT signal of sufficiently high level can not be obtained in the detecting period in which the receiving signal is not synchronized. There are then the substantial disadvantages that a malfunction will occur and that the AFT operation will not be pulled in with plenty of time.